Telephone companies offer customers a number of ways to transport data. One popular way is called Asymmetric Digital Subscriber Line (ADSL). In ADSL, a small portion of the frequency spectrum is used for communicating data from the customer to the central office, and a much larger portion of the frequency spectrum is used for communicating data from the central office to the customer.
Discrete multi-tone (DMT) modulation is used within each portion of the frequency spectrum, i.e., data are carried on equally-spaced carrier signals. The combined number of carrier signals from both portions of the spectrum is implementation-dependent. ADSL implementations that comply with the ITU-T G.992.1 standard have 256 carrier signals, while implementations that comply with the ITU-T G.992.2 standard have 128 carrier signals. Future implementations are expected to have 512 or even 1024 carrier signals (see, e.g., ITU-T G.992.5).
DMT modulation provides for very efficient use of the available communication spectrum because the amount of data carried by each carrier signal is individually customized to fit the signal-to-noise ratio profile of the channel. Each carrier signal is allocated a number of data bits, and the allocation of bits may be dynamically adjusted as channel conditions change. Each carrier signal may also be allocated a small individual gain factor to further improve communications performance.
The allocation of bits and gain factors to carrier signals are typically performed using tables. A gain table includes an individual gain factor for each carrier signal. A bit table includes an individual number of bits allocated to each carrier signal. A tone table may be used to allocate specific data bits to specific carrier signals.
Some channels actually have two signal-to-noise ratio profiles. An example of such a channel is a twisted wire pair in a binder that also carries TCM-ISDN (Time Compression Multiplexing-Integrated Services Digital Network) traffic. TCM-ISDN employs time division multiplexing at a rate of 400 Hz, i.e. the central office alternately transmits data for 1.25 milliseconds, then listens for data from the customer for 1.25 milliseconds. This causes other channels to experience a noise profile that alternates at a rate of 400 Hz.
The interference can be divided into two types: near-end cross talk (NEXT) and far-end cross talk (FEXT). Some confusion can arise when discussing NEXT and FEXT since the meaning of NEXT and FEXT changes with respect to the chosen reference point. For clarity herein, the central office is hereby chosen as the arbitrary reference point, and this reference point will be used consistently throughout. NEXT interference on a given channel is caused by central office transmissions on other channels. FEXT interference on a given channel is caused by transmissions from customers on other channels. TCM-ISDN signaling alternately causes NEXT interference and FEXT interference. The NEXT interference is generally significantly worse than the FEXT interference, although this depends on the distance that the twisted wire pair travels alongside interfering channels.
The ITU-T G.992.1 and G.992.2 standards each address TCM-ISDN interference in their respective Annex C. Two solutions are offered: dual mode solution and FEXT-only solution. In the dual mode solution, two sets of tables (gain, bit, and tone) are used. One set of tables is used to construct symbols for transmission during periods of NEXT interference (“NEXT symbols”), and the other set of tables is used to construct symbols for transmission during periods of FEXT interference (“FEXT symbols”). Although TCM-ISDN signaling uses a 50% duty cycle, it is expected that on average, only 126/340 (about 37%) of the symbols will be free of NEXT interference, and hence constructible as FEXT symbols.
Although the dual mode solution offers a higher data rate, it does add significant cost to the modem in the form of additional memory for the second set of tables. This additional cost is expected to be significant for future ADSL implementations having 512 or more carrier signals due to the increased size of the tables.
The FEXT-only solution is similar to the dual mode solution except that no symbols are constructed or sent during the periods of NEXT interference. Because only FEXT symbols are used, only one set of tables is needed. Although the FEXT symbols typically carry more data than NEXT symbols, sacrificing 63% of the symbols can impose a substantial performance penalty.
Accordingly, it would be desirable to have a memory-efficient method for ADSL transmission in a time-varying noise environment. Such a method would preferably avoid the performance penalty of the FEXT-only solution without suffering the prohibitive expense of the dual mode solution.